A NMR study of the solution structure of the TolR periplasmic domain reveals a C2 symmetric homodimer of mixed helix-sheet secondary structure. The structure has been determined on the basis of a large number of interproton NOEs, supplemented by four different types of backbone residual dipolar couplings, including N-H, Ca-Ha, C'-Ca, and N-C'. Reasonable cross validation statistics and good geometric properties attest to the quality of this structure. The shape of the dimeric structure is in excellent agreement with its SAXS diffraction pattern. Work is continuing on defining the colicin interaction site.